1. Field of the Invention
The present invention relates to a tuning fork and a manufacturing method thereof. More particularly, the present invention relates to a tuning fork produced through the folding of a plate-like material of invariable elasticity and a manufacturing method thereof.
2. Description of the Prior Art
An extremely small-sized tuning fork for use in tuning forks of a piezo-electric type, electromagnetic type and the like is made of a material of invariable elasticity such as elinvar which is superior in an impact characteristic. One method of manufacturing such a tuning fork comprises the steps of punching a material formed into a plate through a rolling operation and folding it. Another method thereof comprises the steps of cutting and machining the material. The tuning fork by the former method will be described later. The tuning fork by the latter method is advantageous in that it can be formed into an optional shape, but is disadvantageous in that the costs for the material and for machining the material is higher. The tuning fork by the former method will be described hereinafter with reference to FIGS. 1 through 4.
FIG. 1 is a perspective view showing a first typical conventional example of such a tuning fork. FIG. 2 is a perspective view illustrating a machining process for the tuning fork. In general, a rolling operation applied to a material gives rise to a residual stress in the rolling direction in the material, which enhances a strength to the bending. Such a tuning fork 10 as shown hereinabove is formed through folding, along a folding line 12, of a blank 11 which has been punched into an elongated rectangular shape extending in a rolling direction as shown in an arrow "a". The portion along the folding line 12 forms a base portion 13 of the tuning fork. Both side portions of the blank form vibrating leg portions 14 and 14 of the tuning fork.
FIG. 3 is a perspective view showing a second typical conventional example. FIG. 4 is a perspective view illustrating a manufacturing process of the tuning fork. A tuning fork 20 shown in FIG. 4 is formed through folding, along a folding line 22, of a U-shaped blank which has been punched such that its end portions extend in a rolling direction as shown by arrow "a". The bent portion forms a base portion of the tuning fork. The bifurcate portions, each extending straightly along its end direction from the base portion 23, form the vibrating leg portions 24 and 24, respectively.
Generally, to improve the quality factor Q of the tuning fork and to stabilize the characteristics such as a time dependent characteristic and impact characteristics, it is preferable to increase the thickness of the base portion of the tuning fork and the strength of the vibrating leg portions. From this point of view, the tuning fork 10 and the tuning fork 20 provide an increased strength of the respective vibrating leg portions, inasmuch as these have the vibrating leg portions 14 and 14 extended in the rolling direction "a". However, in the tuning fork 10 (FIG. 1), the thickness of the base portion 13 of the tuning fork is determined by the thickness "b" of the blank 11 per se, which means that the thickness of the base portion is limited. As a result, the quality factor Q cannot be improved and the characteristics as described hereinabove cannot be stabilized. On the other hand, since in the tuning fork 20 (FIG. 3) the thickness "b" of the base portion 23 can be selected as desired depending on the punched shape of the blank 21, the quality factor Q can be improved and the characteristics as described hereinabove can be stabilized. Nevertheless, as apparent from the shape of the tuning fork 20 and the shape of the blank 21, the tuning fork 20 is disadvantageous in that it is of inferior workability and further results in an increased material loss as compared with the tuning fork 10. Although it is particularly true in the case of an extremely small-sized tuning fork, cracks may be caused near the folding line 22 due to folding in the rolling direction of the material, when the space "c" between the vibrating leg portions 24 and 24 is made small, with the resulting limitation that the space "c" cannot be made small.